1. Field of the Invention
The field of the present invention is that of civil aviation and, in particular, that of the noise generated by airplanes.
2. Description of the Related Art
The noise generated by commercial airplanes, particularly on takeoff, is a nuisance that is widely known and a great many innovations have been conceived in an attempt to reduce it. One of the main sources of this noise is from the jet of the engines, which are used at full power during the takeoff phase. Considerable work has obviously been carried out in an attempt to reduce the noise of the jet of the turbojet engine, for example the fitting of sawtooth chevrons to the exit nozzle, whether the exit nozzle for the hot gases which originate from the primary flow through the engine, or the one for the so-called cold gases which originate from the secondary or bypass flow of the engine.
In order to limit the nuisance to those living or working near airports, strict standards have been imposed that limit the noise that can be perceived at various points situated around the airplane, at various distances and in several directions from the takeoff runway.
One of the particularly critical aspects with which airplane designers have to comply, in terms of the maximum noise that is permissible if the airplane is to be certified, is in a lateral position with respect to the airplane, at a distance of 450 m from the takeoff runway. The presence of the pylon, i.e. of the strut that supports the engine by attaching it to the wing, generates locally, in the region where the gases are ejected, high levels of turbulence in the flow and this results in a very significant increase in the lateral noise of the engine. This phenomenon is particularly keenly felt in configurations in which the pylon protrudes beyond the plane in which the gases are ejected, which is becoming a configuration very often employed in recent commercial airplanes.
The results of numerical calculations or of measurements taken on models in a wind tunnel clearly show that the effects of interactions between the flow flowing around the pylon and the pylon itself generate an appreciable increase in the levels of turbulence and, as a result, in the noise level. A significant change to the angular development of the jet radially around the pylon can also be observed, and this tends to direct the jet from the turbojet engine around the pylon, towards the wing.
Moreover, experience gained demonstrates that the introduction of the pylon, in addition to its having an influence on the increase in noise level in a conventional ejection configuration, can also considerably reduce the effectiveness of other devices that have been installed for reducing the noise of the exhaust gases, such as chevrons or mixers fitted to the exit nozzles. The presence of the pylon also, from an aerodynamic standpoint, modifies the initial development of the jet and therefore its mixing and its acoustic signature.
Thus, in terms of acoustics, the presence of the pylon causes an increase in the ejection noise at the lateral certification point and this increase can vary between 2 and 3.5 EPNdB (Effective Perceived Noise, in decibels) depending on the engine cycle, the size of the pylon and the ejection geometries considered.
Devices, such as those described in patent applications FR 2913401 or GB 2138507 have been conceived in an attempt to control the circulation of air around a pylon supporting a turbomachine. They do not, however, deal with the noise generated on takeoff by the shearing of the velocity between the hot flow or flows and the cold flow of double-flow (bypass) or triple-flow turbojet engines and have no impact on solving the technical problem associated therewith.